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Old 10-02-2016, 11:46 PM   #1
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Tire question for a tire engineer?

Here's the set up-
You have a TT and C rated tires that are right at capacity load wise
You go to D rated for the extra capacity but you don't need that much load ability so you lower the air down to 55 instead of 65 psi (still above the TT weight/tire buy using the available pressure ratings from a major manufacturer.

Would this lower pressure cause enough flex at the tread/sidewall juncture to cause tread separation and tire failure?

Just thinking of the physics and thoughts from other threads here.
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Old 10-02-2016, 11:59 PM   #2
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Not a tire expert but I will give you a real life example.

Truck tires are good for 3195# each at 80 psi
When towing I air them up the rear tires to 80# cause I need the MAX Load Rating
When NOT towing I drop air pressure to 45# cause I don't same weight on them.

Front tires are at 55# all the time due to the less than max load on tires

I typically get 60K before I buy new tires and thread is still good.....I just change them out cause I an anal about tires


MAX Load Rating at MAX Pressure listed on sidewalls of tires is for MAX LOAD
Lower air pressure cna be used as long as it is high enough for the load on tire.

Over loaded/under inflated causes excessive heat...heat destroys tires
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Old 10-03-2016, 12:09 AM   #3
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Check the inflation tables for your tires. As long as you have pressure at or above the level needed for the tire load on your wheels, the sidewalls will not flex too much.

This means you need to know what the tire load is.

Excess pressure above a little margin like 5 psi means your tires are over-inflated which will crown the thread and reduce the tire patch and wear the center of your tires.

Larger tires are a good measure.
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Old 10-03-2016, 08:17 AM   #4
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If they are ST tires, then you should run them at max pressure *especially* if you have a multiple axle trailer. The max pressure helps to prevent the problem you mentioned (flex causing tire failure) and deal with interply shear.

(I am not a tire engineer, but I learned what I know from one. I do hope that one will come in and tell us their opinion, whether that makes me right or wrong)
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Old 10-03-2016, 10:51 AM   #5
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A D rated tire inflated to a C rated tire amount (55#) will only support a C rated load.

You're right back where you started with the C rated tire.
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Old 10-03-2016, 11:23 AM   #6
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I also am not a tire engineer, but my question is why would you want to? It seems that if a tire is built to a certain criteria that to run it at less than criteria would not be good for the integrity of the tire. After all air is free so why not use it?
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Old 10-03-2016, 12:28 PM   #7
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Its a hypothetical question brought about by many threads here on tires
In addition, lots of talk about STs and their failure and specifically "inter-ply shear" failure
Many threads have mentioned loading tire pressure to load on tire by using pressure load tables from a major tire company.
Granted, very low pressures (1/2 normal) will cause issues but will 15% lower cause the same?
Are STs different than LTs in this respect?
Take 205 70 15 STs and compare to 215 70 15 LTs and you see @50# less weight capacity with LTs but at 47 lbs pressure rather than 65 as in D rated STs.
As a corollary, would a lower pressure LT cause less "pounding" inside the TT (absorb more road shock) than a harder 65 lb ST tire?
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Old 10-03-2016, 12:36 PM   #8
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IMO if you load a tire to it's max and apply the max pressure you will get a tire patch on the ground and a resultant tire flex.

If you load a tire under it's capacity and inflate it to the recommended pressure you will get the same tire patch on the ground and the same tire flex.
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Old 10-03-2016, 12:41 PM   #9
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Quote:
Originally Posted by joshuajim View Post
A D rated tire inflated to a C rated tire amount (55#) will only support a C rated load.

You're right back where you started with the C rated tire.

This is the best advice. The air carries the load, not the sidewall. Tandem tires share the load. If one blows the other is instantly overloaded because it's carrying twice what it was. If it's under-inflated, it's even more overloaded.
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Old 10-03-2016, 12:51 PM   #10
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Quote:
Originally Posted by Cliffy View Post
... using the available pressure ratings from a major manufacturer.
If the table you have is from the same manufacturer that made your tires, use it. If it isn't, who is to blame for not properly inflating that particular tire you are using?
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Old 10-03-2016, 07:37 PM   #11
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Thing to remember is the load on all tires change as you go down the road due to bumps, curves, wind shear, etc.
My tires have a max sidewall of 80psi, and thats what they get.
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Old 10-06-2016, 09:26 PM   #12
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Quote:
Originally Posted by timetogo View Post
Thing to remember is the load on all tires change as you go down the road due to bumps, curves, wind shear, etc.
My tires have a max sidewall of 80psi, and thats what they get.
More good advise!
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Old 10-07-2016, 10:05 PM   #13
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The tire expert is your tire manufacturer. ST tires are designed to be operated at maximum sidewall pressures. They run cooled and with less stress. There is no mileage warranty to worry about and their life expectancy is somewhere between 3-5 years.
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Old 10-08-2016, 10:18 AM   #14
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According to these charts ST tires can be inflated to various pressure depending upon the load.

http://www.goodyearrvtires.com/pdfs/rv_inflation.pdf
Trailer Tire Load/Inflation Chart | Maxxis Tires USA
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